An advantageous base material for polyethylene cross-linked pipes (PEX pipe) is a narrow molecular weight distribution (nMWD) polyethylene with high crosslinking potential and low content of odour active components. High reactivity towards crosslinking can be achieved by ensuring a high degree of unsaturation in the polymer.
It is known that nMWD can be achieved by Single Site Catalyst (SSC) technology but normally these polymers have a relatively low crosslinking potential compared to for example, chromium catalyst produced polyethylene (Cr-PE). Cr-PE contains a large number of unsaturations making it readily cross-linkable.
In single site catalysis however, it is normal to control molecular weight using hydrogen and this makes a significant difference to the structure of the polymer. When a propagating PE molecule is terminated by hydrogen, the polyethylene molecule is saturated at the terminus and hence there are low levels of unsaturation in the formed polymer. It is therefore difficult to increase the content of unsaturations in SSC PE resins.
One way of introducing unsaturation into the backbone of a single site produced polyethylene is to employ dienes or other multifunctional compounds as comonomers. However, these comonomers have rather high odour, low reactivity, and residual unreacted comonomers maybe hard to remove, causing unacceptable organoleptic properties.
The conventional route therefore to achieving highly unsaturated polymers is by employing a particular catalyst which is autoterminating, e.g. Cr catalysts. Polymers made using Cr catalysis typically have one unsaturation per polymer molecule. It is known, however, that these catalysts give broad molecular weight distribution polyethylenes (e.g. Mw/Mn values of much greater than 10) which are not favoured in the crosslinking process and, as they form oligomers, which are associated with unacceptable organoleptic properties.
One solution might be therefore to use the single site materials and tolerate the low level of unsaturation present and simply add more cross-linking agent to achieve the necessary cross-linking degree. However to cross-link such a polymer requires much higher levels of cross-linking agents or doses of radiation to achieve a certain degree of X-linking. This has cost, odour and safety implications and is not favoured.
There remains therefore a need to produce single site polyethylene (SSC PE) resins for PEX pipe applications but with increased levels of unsaturation to mimic chromium polyethylene like unsaturations but with narrow molecular weight distribution and good organoleptic properties. The polymer should also possess good processability and pipes formed from the polymer should have advantageous torque properties.
WO2010/049170 discloses single site catalysed polymers with Mw/Mn values of less than 5 but hydrogen is used in their formation. The absence of hydrogen is important during manufacturing to achieve the necessary unsaturation content which forms part of the invention. Hydrogen is used both in loop and gas phases in the examples of WO2010/049170.
US2003/0130445 describes polypropylene polymers which may contain ethylene as a comonomer but these polymers are not polyethylenes which must have a high ethylene content.
The abstract of JP03-099832 discusses the formation of crosslinked polyethylene pipes using a radical generating agent, silane compound and powdery polyethylene where the number of double bonds per 1000 C atoms is 0.5 or more. The Mw/Mn values are not reported as being low as required herein.
U.S. Pat. No. 5,446,221 is not in the field of pipes but the polymers therein are characterised by a high terminal ethenylidene type unsaturation in which 95% or more of the polymer chains possess such an unsaturation. The ethylene content of the polymers in U.S. Pat. No. 5,446,221 is however, 20 to 80 wt % and these polymers are therefore rubbers. At the comonomer content levels in U.S. Pat. No. 5,446,221, very low densities will be exhibited.